1. Field of the Invention
The invention relates to a binary burner for an auxiliary motor vehicle heater, for combustion of liquid fuel in the presence of combustion air, with a combustion chamber which is downstream of a nozzle for atomization of liquid fuel, and a Venturi nozzle for atomization of liquid fuel.
2. Description of Related Art
German Patent DE 41 18 538 C2 discloses an example of a burner of the initially mentioned type. This burner is based on a dual-fuel nozzle for atomization of fuel which has a relatively complicated structure. In particular, this nozzle consists of an essentially cylindrical mixing chamber, a feed pipe coaxial to its lengthwise axis for the liquid fuel, and a combustion air medium feed. The mixing chamber of the nozzle is closed by a nozzle insert for discharge of the atomized fuel/combustion air mixture and has a central nozzle hole and a conical inner bevel which converges in the flow direction. The combustion air feed consists of a feed pipe which tangentially penetrates the mixing chamber wall, dips into it and discharges at a greater height above the nozzle insert than the feed pipe for the liquid fuel. Due to this complex structure of the dual-fuel nozzle, for its proper operation for atomization of the liquid fuel, a considerable air overpressure is required which must be applied by a correspondingly powerful fan.
In conjunction with the gas burners, use of Venturi tubes as a mixing means is known. Furthermore, U.S. Pat. No. 4,396,372 discloses a burner system in which liquid fuel in the form of kerosene is delivered by means of a nozzle to a vaporizer provided with electrical heating means, with a Venturi-shaped inlet area.
One object of this invention is to devise a burner or a Venturi nozzle for atomization of the liquid fuel which does not require a high air overpressure for atomization of the liquid fuel and which operates reliably.
This object is achieved by a burner with nozzle having a Venturi tube or a Venturi nozzle with an inlet part which tapers in the flow direction of the combustion air and with a diffuser part which widens in the flow direction of the combustion air, the larger diameter end of the diffuser part discharging the liquid fuel into the combustion chamber, in the underpressure area in the transition area from the inlet part to the diffuser part, and by a Venturi nozzle that is axially divided into a fuel/combustion air discharge part and a fuel/combustion air supply part, the two Venturi nozzle parts being heat insulated relative to one another.
Accordingly, in accordance with the invention, the complex nozzle used for the binary burner is replaced by an arrangement with a Venturi tube or a Venturi nozzle, which to prevent coking and heat losses is made at least partially of a ceramic material. As a result of the pressure recovery of the Venturi tube or the Venturi nozzle it is not necessary to deliver air with a high overpressure so that the strong fan or additional fan which had been necessary in the past for this purpose can be omitted. The Venturi tube (or Venturi nozzle) used in accordance with the invention, moreover, has the advantage that economical production is possible.
Both the installation cost as well as the production costs are advantageously reduced if at least the front section of the Venturi diffuser part or the Venturi nozzle is made integral with the combustion chamber.
In the simplest case, the diffuser part of the Venturi tube or Venturi nozzle is formed with a uniform opening angle. However, according to one advantageous development, this Venturi tube or Venturi nozzle diffuser part can have sections of different opening angles, the section with the greatest opening angle bordering the combustion chamber.
Advantageously, the fuel is supplied to the Venturi tube or Venturi nozzle via a fuel feed tube with downstream end projecting into the Venturi tube. This downstream end of the fuel feed tube can discharge into a downstream dual-fuel nozzle or into the underpressure area of a second smaller Venturi tube which is operated in turn by the pressure drop of the Venturi tube or Venturi nozzle and ends in its underpressure region in order to achieve pre-atomization. Furthermore, the fuel feed tube is advantageously located within the Venturi tube or Venturi nozzle running coaxially to its lengthwise center. The downstream end can be located at different locations of the Venturi tube or Venturi nozzle, for example, in the inlet part or in the diffuser part of the Venturi tube or the Venturi nozzle or at its narrowest site between the inlet part and diffuser part.
Furthermore, it is advantageously provided that the combustion chamber have at least one additional inlet for secondary air. This secondary combustion air inlet is preferably located in the plane of the combustion chamber in which the Venturi tube or Venturi nozzle discharges.
The invention is explained below by way of example using the drawings.